AW: [EFM-P2P][EFM] PMD considerations
Hi Ulf,
Nice discussion points.
I would have another viewpoint on some of the
issues. For one, I would put the cross over point for single vs dual
fibre solutions at one to two kms. These are the costs of the optics alone.
You are right that it is more problematic to achieve the requires
return loss with a single fibre, single wavelength solution. However,
this is a design issue and this solution is in our opinion the most
cost effective solution for P2P. A power range of 10 dB is
unlikely so I don't see the need for such a high return loss.
In terms of the yield/cost case for single/dual fibre solutions, the
bi-directional component is more complex and is therefore more expensive.
However, the factors effecting yield are the same for both so this is
not the deciding issue.
Regards
Tom
-----Ursprüngliche Nachricht-----
Von: Ulf Jönsson F (ERA) [mailto:Ulf.F.Jonsson@xxxxxxxxxxxxxxx]
Gesendet am: Montag, 17. Dezember 2001 20:49
An: stds-802-3-efm-p2p@xxxxxxxxxxxxxxxxxx;
stds-802-3-efm@xxxxxxxxxxxxxxxxxx
Betreff: [EFM-P2P][EFM] PMD considerations
Hi all,
The following text brings up some considerations regarding the EFM optical PMD from a component perspective. It has been written with great help from our Ericsson internal experts on the optoelectrical component side.
For the physical medium, i.e. the O/E-converters and the fiber connecting them, a few aspects may be high-lighted:
1. Data rate
2. Single or multimode fiber
3. Single or dual fiber
We will discuss these aspects in more detail and will also try to draw a conclusion. Hopefully this will start a discussion on the reflector that may make it easier for us to agree on a (few) baseline proposal(s) in March.
1. Data rate
------------
The choice is between 100Mbps and 1000Mbps. Of course one must pay a premium for a tenfold speed increase, throughout the entire system (A more detailed cost analysis will be presented at the January interim). Optimizing an O/E converter design for 100Mbps instead of 1000Mbps means
* inherent improvement of receiver sensitivity.
* lowered demands on output optical power (consequence of above).
* lowered demands on thermal management (both inherent, due to lower speed, and consequence of above)
* lower crosstalk
All these factors will facilitate the module design, simplify the assembly and increase the yield, thus substantially lower the costs. The argument for 1000Mbps, that the higher volumes for this product will yield lower cost, neglect the impact of EFM as a cost driving application itself. This application should in itself be enough to create sufficient production volumes. Thus, it does not seem optimal to let the vast majority of connections where 100Mbps is sufficient pay that cost premium, especially as a P2P topology allows for relatively easy individual line upgrades. On the other hand, 1000BASE-X will in a P2P topology be appropriate for premium subscribers and for aggregate traffic higher up in the access network and it will of course be appropriate to use in a P2MP network. Hence, we see a need to include both a 100Mbps PMD and a 1000Mbps PMD in EFM.
2. Singlemode or multimode fiber
--------------------------------
As of now, multimode systems are significatly more low-cost than singlemode systems. Though this difference will decrease as the singlemode component volumes increase, a certain difference will always remain, due to the less stringent geometrical tolerances in a multimode system. For those applications where multimode systems are appropriate, there is no need to pay the singlemode premium. What is important is that a large number of connections require singlemode systems, both due to present distance limitations and to future upgradeability.
3. Single or dual fiber
-----------------------
O/E converters for a single fiber system are inevitably more expensive than those for a dual fiber system, due to the higher complexity. Just as inevitable is the fact that this difference will be more than compensated at very long link lengths. The question is the cross-over distance, and the distribution of potential installations below and beyond this cross-over, respectively. If it is regarded necessary to include both options in the standard, how can that be made with a minimum of effort? Let us examine the implications on the basic parameters.
3.1. Power budget
A dual fiber system can, and should, allow for a wide output power range, in order to achieve high production yields in a low-cost assembly process.
If wavelength separation is used in the single fiber case, the power specification should be equal for dual and single fiber. The extra attentuation caused by the splitters are hidden inside the converters, and just has to be compensated for by extra laser power and increased internal receiver sensitivity, respectively.
The single wavelength case is more difficult, due to constraints imposed by the reflection crosstalk. In order not to have completely unrealistic back-reflection demands, the span of the allowed output power must be minimized. Otherwise, the transmitted power from a "low-end" module would drown in the reflected power from a relatively high power module. Assume e.g. a power span of 10dB, a link budget of 10dB and a required SNR of 10dB. This implies a total allowed near-end reflection of below -30dB, which is not easily achieved.
Thus, if the output power range for dual fiber is e.g. -5dBm to -15dBm, the single fiber version should probably be a part of that, something like -12dBm to -15dBm.
3.2. Wavelength
For dual fiber systems, the operating wavelegth window can, from a component perspective, be selected freely within the SM fiber window 1300-1600mn. A wavelength separated single fiber system of course have strict requirements regarding this matter. For such a system it is also required to have two types of transceivers, for each end of the connection. Depending on the actual implementation of the components for a single wavelength single fiber system, some wavelength restrictions could be needed, as the splitters might have a wavelength dependence.
3.3. Connectors
For dual fiber, several types of standard connectors should be allowed, e.g. MT-RJ, LC, MU, etc. The requirements on connector performance can be kept low, to reduce costs, since the desired power budget is easily achieved, and there is no back-reflection problem.
The same should be valid for single fiber WDM systems, even though the power budget is a bit harder to meet in this case. Possibly the connector attenuation must be a bit tighter specified.
For non-WDM single fiber, the crosstalk problem make low reflection connectors necessary throughout the entire system.
Conclusion
----------
Eight different P2P relevant configurations, each with its own merits and drawbacks, can be distinguished. These are:
100 Mbps MMF dual fiber
100 Mbps SMF dual fiber
100 Mbps SMF single fiber
100 Mbps SMF single fiber WDM
1000 Mbps MMF dual fiber
1000 Mbps SMF dual fiber
1000 Mbps SMF single fiber
1000 Mbps SMF single fiber WDM
of these three already exist as standards within IEEE 802.3, namely
100 Mbps MMF dual fiber
1000 Mbps MMF dual fiber
1000 Mbps SMF dual fiber
100 Mbps SMF dual fiber is at present not an Ethernet standard. Still, components exist and are used when needed. ANSI has standardized a PMD for 100Mbps FDDI over SMF (ANSI X3.184-1993). The corresponding FDDI standard for MMF is used as a reference for Ethernet 100BASE-FX.
The need to incorporate 100 Mbps SMF dual fiber within the Ethernet family is obvious. Since it also seems to be the most appropriate choice for a large number of EFM connections, it should be the first choice for an EFM PMD. This PMD should of course to a large extent be based on the 100BASE-FX, with the physical media specifications optimized for low-cost components with sufficient performance.
To give a variety of options, it seems reasonable to also incorporate 100Mbps MMF dual fiber as well as 1000Mbps dual fiber in EFM. As already being Ethernet standards, this should be possible without much extra work.
Single fiber systems are a bit more complicated, since the requirements are more closely connected to the actual implementation, and a PMD are more different from existing standards. One way to go, since the requirements (with wavelength for the WDM solutions as a possible exception) is within the dual fiber specification, only tighter specified, would be to use the dual fiber PMD as a base and have different categories within that. These could be one or two single fiber options, but also extended temperature and extended range dual fiber options. Depending on the progress of the work, the single fiber options can either be tightly defined within the base PMD, or kept rather open for different manufacturer implemenations. The important issue is to let the time-schedule be set by the most straightforward, dual fiber, solution.
Best regards,
Ulf Jönsson & Hans Mickelsson
Ericsson